Welcome nannymo!
I just did a little map research on Google.
Billie leaves home at 6:30
She's on Hwy 208 from 6:32 to 6:55 (assuming they're doing the speed limit) and at her desk at Cogdell Memorial in Snyder by about 7 a.m.
Shawn leaves Weaver Pipeline Services in Snyder at 6:10
He's on Hwy 208 from about 6:15 to 6:35 p.m. when he gets back to CC city (assuming he's doing the speed limit)
Hwy 208 is the direct route for both of them. If both cars were doing the speed limit, they would have crossed paths when he was just getting off the 208 and Billie was getting on it. If he was driving much faster than the speed limit, I suppose he could have stopped at his grandma's in Dunn, knocked for a very short time, and gotten gotten off the 208 at around 6:30, just a minute or two before he would have passed Billie. But, he has to be near Billie's in CC from 6:35 to 6:56 per cell phone pings that hit off the same phone as Billie's house.
So, if he just exited the 208 onto the I-20 and drove straight through CC, what took him 21 minutes to get out of the CC cell tower range? I can't imagine he would be pinging off that same CC tower for 21 minutes headed down I-20 to Big Spring?. CC is a small city from what I understand, so it shouldn't be more than several minutes before he'd be out of that cell radius (I assume?). I think he had to have been doing something in CC for at least 10 minutes..
So, really, Billie's time line for SA on 12/27 a.m. is implausible to me when you consider the cell pings.
Note: I think tehcloser did something similar and more refined a few days ago when I was on and off the board. I'm gonna see if I can find her post for my own reference.
(my times are based on speed limits used for Google driving directions, so they're approximations only since we don't know how fast they were driving or if there was any traffic).